1. Field of the Invention
The invention relates generally to methods and apparatus for providing inserts for use in roller cone drill bits that have improved properties when compared with prior art inserts.
2. Background Art
Drilling in the earth is commonly accomplished by using a drill bit having a plurality of rock bit roller cones (“cutter cones”) that are set at angles relative to the drill string axis. The bit essentially crushes the formations through which it drills. The roller cones rotate on their axes and are, in turn, rotated about the main axis of the drill string. In drilling boreholes for oil and gas wells, blast holes, and raise holes, rock bit roller cones constantly operate in a highly abrasive environment. This abrasive condition exists during drilling operations even with the use of a medium for cooling, circulating, and flushing the borehole. Such a cooling medium may be either drilling mud, air, or another liquid or gas.
One type of commonly used rock bit contains a plurality of inserts (“cutting elements”) which are press-fit into the body of the cone. These inserts may be formed from a variety of materials, such as tungsten carbide, or other hard materials. The inserts are retained in “cutter pockets” (holes in the cone body) by the interference between the walls of the cutter pocket and the sides of the insert.
The inserts are subjected to a number of different forces that cause the inserts to be forcibly ejected from the insert pockets. One solution, therefore, to increasing drill bit life is to increase the amount of force required to push an insert from an insert pocket.
Other traditional methods for improving the “push out force” include increasing the size of the insert, relative to the pocket (to increase the interference), or conversely, decreasing the size of the pocket. However, such prior art methods have inherent limitations, because as the size of the pocket is decreased, or the cutter size is increased, at some point cone cracking, or yielding of the area around the cutter pocket occurs.
As used herein, the “push out force” is a measure of the force required to physically displace the insert from a selected position. Those having ordinary skill in the art will recognize that the push out force may be measured in a number of different ways, and no limitation on the scope of the invention is intended by the discussion provided below.
FIG. 1 illustrates a typical prior art rock bit for drilling boreholes. The rock bit 10 has a steel body 20 with threads 14 formed at an upper end and three legs 22 at a lower end. Each of the three rolling cones 16 are rotatably mounted on a leg 22 at the lower end of the body 20. A plurality of cemented tungsten carbide inserts 18 are press-fitted or interference fitted into insert sockets formed in the cones 16.
When in use, the rock bit is threaded onto the lower end of a drill string (not shown) and lowered into a well or borehole. The drill string is rotated by a rig rotary table with the carbide inserts in the cones engaging the bottom and side of the borehole 25 as shown in FIG. 2. As the bit rotates, the cones 16 rotate on the bearing journals 19 and essentially roll around the bottom of the borehole 25. The weight on the bit is applied to the rock formation by the inserts 18 and the rock is crushed and chipped by the inserts. A drilling fluid is pumped through the drill string to the bit and is ejected through nozzles 26 (shown in FIG. 1). The drilling fluid then travels up the annulus formed between the exterior of the drill pipe and the borehole 25 wall, carrying with it most of the cuttings and chips. In addition, the drilling fluid serves to cool and clean the cutting end of the bit as it works in the borehole 25.
FIG. 2 shows the lower portion of the leg 22 which supports a journal bearing 19. A plurality of cone retention balls (“locking balls”) 21 and roller bearings 12a and 12b surround the journal 19. An O-ring 28, located within an O-ring groove 23, seals the bearing assembly.
The cone includes multiple rows of inserts, and has a heel portion 17 located between the gage row inserts 15 and the O-ring groove 23. A plurality of protruding heel row inserts 30 are about equally spaced around the heel 17. The heel row inserts 30 and the gage row inserts 15 act together to cut the gage diameter of the borehole 25. The inner row inserts 18 generally are arranged in concentric rows and they serve to crush and chip the earthen formation.
What is needed therefore, are methods and apparatus for improving the working life of drill bits.